1. Field of the Invention
This invention relates to current mirrors, and more particulary to current mirrors for depletion-mode silicon or gallium arsenide (GaAs) field effect transistor (FET) circuits.
2. Description of the Related Art
A current mirror is a circuit in which the output current tracks the input current, and the output current is either equal to or a fixed multiple of the input current. A current mirror is a standard building-block in the design of analog circuits. It functions essentially as a current amplifier, with a low input impedance and a high output impedance.
Current mirrors are known for use with bipolar transistors and enhancement-mode Metal Oxide Semiconductor Field Effect Transistors (MOSFETs). However, these current mirrors cannot be used with depletion-mode gallium arsenide FET devices because standard current mirrors operate on the assumption that the transistor is in its active mode when its gate is connected to the drain of the transistor. This is true for enhancement-mode Field Effect Transistors (FETs), but is not true for gallium arsenide or other types of depletion-mode transistors. In a depletion-mode gallium arsenide or other type of depletion-mode Field Effect Transistor, the gate potential must be at a lower potential than that of the drain to be in the active mode.
All current mirrors work by forcing an input current through the drain of the transistor, and measuring the resulting gate potential. That gate potential is used to control the gate potential of another transistor to create an output current. There is no precise, known technique for forcing current through the drain of a depletion-mode FET and still keeping its gate potential lower than that of the drain. The standard current mirroring techniques are described in Sedra Adel S. et al, Microelectronic Circuits, (1982), Holt, Rinehart and Winston, San Francisco, pages 433-437.
A current mirror for depletion-mode gallium arsenide devices is desirable because of the advantages of GaAs over silicon in semiconductor devices. A gallium arsenide MESFET can switch about twice as much current as a silicon MESFET with a comparable geometry. Since the gate-source capacitances for gallium arsenide and silicon MESFETs are comparable, the gallium arsenide switch is about twice as fast as the silicon device.
Also, the turn-on and turn-off times are two to three times lower (faster) or gallium arsenide MESFETs than for comparable silicon MESFETs. See Nowogrodzki, M. (editor), Advanced III-V Semiconductor Materials Technology Assessment (1984), Noyes Publications, Park Ridge, N.J., pages 95-113. A good introduction to gallium arsenide technology is Ferry, David K., Gallium Arsenide Technology (1985), Howard W. Sams and Company, Inc., Indianapolis.
A basic current mirror using enhancement-mode MOSFET devices is shown in FIG. 1. In FIG. 1 an enhancement-mode MOSFET T1 has its drain connected to mirror input 10. The drain of transistor T1 is connected to its gate via line 12. The gate of transistor T1 is connected to the gate of transistor T2 via line 14. The drain of transistor T2 is connected to mirror output 16. Since the gate of transistor T2 is connected to the gate of transistor T1, the current through transistor T2 and output at mirror output 16 will be equal to or "mirror" the current through transistor T1 and input at mirror input 10 if transistors T1 and T2 are identical.
More specifically, the drain current for a FET--both gallium arsenide and silicon--may be approximated by the following equations: ##EQU1## where: I.sub.D is the drain current;
k is the current gain factor, which is proportional to the width of the transistor; PA1 V.sub.GS is the gate-source voltage; PA1 V.sub.T is the threshold voltage; PA1 V.sub.DS is the drain-source voltage; and PA1 .lambda. is the channel length modulation parameter.
In FIG. 1, the current flowing through the drain of transistor T1 will be the circuit's input current, I.sub.in. If we assume that .lambda. is small, then the gate to source voltage V.sub.GS of transistor T1 is (I.sub.in /k.sub.1) 1/2 +V.sub.T, where k.sub.1 is the current gain factor for transistor T1. The gate to source voltage of transistor T2 is the same as for transistor T1 Thus, V.sub.GS of transistor T2 will be (k.sub.2 /k.sub.1)I.sub.in, which is equal to the output current I.sub.out. This equation gives the basic principle behind most existing current mirrors and amplifiers. Unfortunately, this current mirroring technique will not work for depletion-mode FET technology, because it assumes that transistor T2 is in the saturation mode (e.g. V.sub.DS &gt;V.sub.GS -V.sub.T) when the gate of transistor T1 is connected to its drain. This not true for depletion-mode technology, where the threshold voltage V.sub.T is negative.
FIG. 2 depicts a so-called "Wilson" current mirror that is more precise than the typical current mirror depicted in FIG. 1. In the Wilson approach, an additional FET T3 is connected to the mirror output 16. The gate of transistor T3 is connected to mirror input 10 via line 20. The drain of transistor T2 is connected to its gate via line 18. The purpose of transistor T3 is to increase the linearity of the current mirror output by increasing the mirror's output impedance. However, the Wilson current mirror depicted in FIG. 2 still may not be used with depletion-mode FET devices for the reasons discussed above.